The video vectorscope is an instrument that is widely used for evaluation of a composite color television signal. As used in this description and in the appended claims, the term "vectorscope" means an instrument having an input terminal, a display surface, means for generating a visible dot on the display surface, X and Y deflection means for deflecting the position of the visible dot in mutually perpendicular rectilinear directions, a wave regenerator for generating a continuous wave signal at a predetermined operating frequency, first and second demodulators having their outputs connected to the X and Y deflection means respectively and each having first and second inputs, means connecting the output of the wave regenerator to the first inputs of the first and second demodulators with a quarter-period relative phase difference, and a filter which passes signal components at the operating frequency of the wave regenerator and is connected between the input terminal of the vectorscope and the second inputs of the first and second demodulators. The term "video vectorscope" means a vectorscope in which the operating frequency is the color subcarrier frequency.
A composite color video signal contains timing information and information representative of the distribution of color over a scene. The scene may be a natural scene, imaged on the image-receiving surface of a video camera, or it may be an artificial scene, such as might be created using a video graphics unit or a test signal generator. In any event the signal, when used to drive a video display unit, causes the video display unit to create an image that conveys intelligible information through the visual sense. As used in this description and in the appended claims, a signal is "representative of a variable other than the distribution of color over a scene" if, when used to drive a video display unit, it does not cause the display unit to create an image that conveys intelligible information through the visual sense. An image conveys intelligible information through the visual sense if it contains not only information representative of color difference but also information representative of perceptible structure.
It is common for a videotape recorder (VTR) to include a video vectorscope in its instrument bridge. The vectorscope is used to determine whether the color information of a composite color television signal being processed by the VTR is properly encoded, so that upon playback the color information can be recovered using a standard display.
A VTR is used to record not only visual information but also audio information. Frequently, an audio signal is transmitted about a television studio in balanced form using a two-conductor cable. With a monaural audio system, the relative polarities of the two conductors that carry the balanced audio signal are unimportant. Consequently, in the case of a monaural audio system it is not necessary to pay attention to the polarities of the two conductors, and many of the connectors used for connecting the two-conductor cables are not polarized.
With the increasing use of stereophonic audio systems in television studios, it has become necessary to distinguish the polarities of the conductors, of a two-conductor audio cable, because if the left audio signal is out of phase with the right audio signal, when the signals are combined to produce L+R and L-R components information that should be added will be subtracted and vice-versa. It is therefore necessary to provide an instrument that will enable a determination to be made easily regarding whether the two balanced cables of a stereophonic audio system are connected with the proper polarities.
An X-Y oscilloscope may be used to determine whether two periodic signals are in phase, by connecting the two signals to the two deflection amplifiers respectively and observing the shape of the display that is obtained. If the two signals are pure sine waves, the display will be a Lissajous figure, and its shape will depend on the phase and frequency relationships between the two signals. If the signals are the same frequency, the Lissajous figure will be an ellipse having a major axis extending diagonally across the screen of the CRT from its lower left corner to its upper right corner if the signals are in phase. If the signals are out of phase, the major axis of the ellipse will be disposed along the other diagonal of the CRT screen. It has been proposed that this type of display be used to determine whether the cables of a stereophonic audio system are connected to a VTR with the proper polarity. However, the space available on the instrument bridge of a VTR is severely restricted, and addition to the bridge of an instrument to check the polarities of the audio connections to the VTR may necessitate removal of some other instrument.